I've had similar problems with usb - apparently qemu is unable to restore usb
controllers. Perhaps this is a similar issue - I don't know. If that is the
issue, hopefully such things will be fixed at some stage.

Perhaps.. But there is something really amazing!! I use ethereal to
listen communication and I can show the icmp request and the arp request
coming on each interfaces but the nothing append the virtual machine do
not reply... I don't understand... It's a riddle!!! Can someone help me?